J . A m . Chem. SOC.1982, 104, 4865-4878
4865
Synthesis and Properties of [ (r-C5H5)Re(NO)(PPh3)(=CHC6H5)]+PF6-: A Benzylidene Complex That Is Formed by a Stereospecific a-Hydride Abstraction, Exists as Two Geometric Isomers, and Undergoes Stereospecific Nucleophilic Attack William A. Kiel,la Gong-Yu Lin,lPAnthony G. Constable,’P Fred B. McCormick,’a Charles E. Strouse,l’ Odile Eisenstein,*lb3*and J. A. G l a d y ~ z * ’ ~ , ~ Contribution from the Departments of Chemistry, University of California, Los Angeles, California 90024. and University of Michigan, Ann Arbor, Michigan 481 09. Received February 26, 1982 Abstract: Reaction of (q-C5H,)Re(NO)(PPh3)(CH2C6H5) (1) with Ph3C+PF6-at -78 ‘C gives the benzylidene s c - [ ( q C5H5)Re(NO)(PPh3)(=CHC6H5)]+PF6(2k), which isomerizes upon warming ( t l I 2= 17 min at 29.5 OC, AH* = 20.9 f 0.4 kcal/mol, L S * = -3.8 f 0.2 eu) to a new Re=C geometric isomer, ac-[(q-C5H5)Re(NO)(PPh9)(=cHc6H5)]+PF6(2t). The 2t/2k equilibrium mixture is 299:1, but irradiation between -78 and -20 OC establishes a (55 f 3):(45 f 3) photostationary state. The structures of 2t and 2k are confirmed by X-ray crystallography (2t) and extended Hiickel MO calculations. Nucleophiles (Nu) Li(C2H5)3BD,CH,Li, CH3CH2MgBr, C6H5CH2MgCl,PMe,, and CH30Na attack 2t to give diastereomerically pure adducts (q-C,H,)Re(No)(PPh,)(c~uc6H~), in which a new chiral center is stereospecifically generated. The same nucleophiles react stereoselectively with 2k, affording adducts in (generally) 92-95:8-5 diastereomer ratios. The minor diastereomers from 2k are identical with the products from 2t. Thus nucleophilic attack upon 2t and 2k occurs preferentially from the same direction. An X-ray structure of (SS,RR)-(q-C5HS)Re(NO)(PPh3)(CH(CH2C6HS)C6H5) (5t, from C6HsCH2MgClattack upon 2t) establishes this direction to be antiperiplanar to the PPh3. When (SS,RR)-(q-C5H,)Re(NO)(PPh3)(CHDC6H5) (l-a-dl-t; from Li(C2H&BD attack upon 2t) is treated with Ph3C+PF6-,exclusive H- abstraction occurs to give first tk-a-d, and then 2t-a-dl. When (SR,RS)-(q-C5H,)Re(NO)(PPh,)(CHDC6H5) (l-a-d,-kfrom Li(C2H5),BHattack upon 2t-a-dl or Li(C2H5)3BD attack upon 2k) is treated with Ph3C+PF;, preferential (92:8) D- abstraction occurs to give undeuterated 2k and (subsequently) 2t. These data (after an isotope effect correction) indicate that thepro-R a-hydrogen of 1 is abstracted by Ph3C+PF6-essentially stereospecifically. Insights into this specificity are gained by IH NMR, extended Hiickel MO calculations, and the chemospecific (SR,RS: 7t; SS,RR: 7k) by Ph3C+PFc. and stereospecific abstraction of CH30- from (yC5H5)Re(NO)(PPh3)(CH(OCH3)C6H5) A mechanism is proposed in which the least stable rotamer of 1 is the most reactive toward Ph3C+PF6-.
Introduction During the last decade, the use of chiral transition-metal catalysts in asymmetric organic synthesis has received increasing a t t e n t i ~ n . ~Extremely useful stoichiometric reagents such as tartrate system for Sharpless’ Ti(O-i-Pr)4/t-BuOOH/diethyl asymmetric epoxidation have also been d e ~ e l o p e d . ~However, understanding of the mechanistic basis for chirality transfer is still at a very formative stage. Some surprising results have been obtained from initial studies. For instance, the [Rh(S,S-chirapho~)S’~]+-catalyzed~ asymmetric hydrogenation of prochiral enamides p r d s via the less stable of two diastereomeric olefin complexes. We recently described syntheses of rhenium alkyls and alkylidenes of the formulas (q-C,H,)Re(NO)(PPh,)(CH,R) and [(q-C5H5)Re(NO)(PPh3)(=CHR)]+PF6-.’ Both classes of compounds contain a pseudotetrahedral asymmetric rhenium and have been found to undergo unprecedented ligand-based stereospecific (or highly stereoselective) stoichiometric reactions. These transformations offer the opportunity to study the transfer of metal-centered chirality to carbon at a fundamental level and consequently have been the subject of detailed investigations in our laboratory. (1) (a) UCLA. (b) University of Michigan. (2) Address all correspondence on the MO calculations to this author. ( 3 ) Address other correspondence to this author at the Department of Chemistry, University of Utah, Salt Lake City, UT 84112; Fellow of the
Alfred P. Sloan Foundation (1980-1984) and Camille and Henry Dreyfus Teacher-Scholar Grant Recipient (1980-1985). (4) (a) Kagan, H. B.; Fiund, J. C. Top. Stereochem. 1978, 10, 175. (b) Fryzuk, M. D.; Bosnich, B. J. Am. Chem. Soc. 1979,101,3043. (c) Valentine, D., Jr.; Scott, J. W. Synthesis 1978, 329. ( 5 ) Katsuki, T.; Sharpless, K. B. J . Am. Chem. SOC.1980, 102, 5974. (6) Chan, A. S.C.; Pluth, J. J.; Halpern, J. J . Am. Chem. SOC.1980, 102, 5952. Chua, P. S.;Roberts, N. K.; Bosnich, B.; Okrasinski, S.J.; Halpern, J. J . Chem. SOC.,Chem. Commun. 1982, 1278. Chiraphos = (SS,RR)(C~HS)ZPCH(CH~)CH(CH,)P(C~HJ)~. (7) Kiel, W. A.; Lin, G.-Y.; Gladysz, J. A. J . Am. Chem. SOC.1980, 102, 3299.
0002-7863/82/1504-4865$0l.25/0
Scheme I. Syntheses and Interconversions of Isomeric Benzylidene Complexes’o
synclinal
anticlinal
In this paper, we present a full account of our work with the benzyl complex (q-C5H,)Re(NO)(PPh,)(CHzC6H5) (1) and the benzylidene complex [ (q-CsH5)Re(NO)(PPh,)(=CHC6H,)]+PF6(2).’+’ As will be disclosed, three principal findings emerge from our chemical data: (a) Reaction of (q-C,H5)Re(NO)(PPh,)(CHzC6Hs) (1) with Ph3C+PF6-results in abstraction of only one of the two diastereotopic a hydrogens. (b) The product from a, benzylidene 2, is generated initially as a ”kinetic” Re=C geometric isomer (2k); at 10-25 OC,2k isomerizes to a “thermodynamic” geometric isomer (2t). ~
~
~~~~
(8) Constable, A. G.; Gladysz, J. A. J . Orgunomet. Chem. 1980, 202, C21. (9) McCormick, F. B.; Kid, W. A.; Gladysz, J. A. Orgunometallics 1982, 1 . 405.
0 1982 American Chemical Society
4866 J . Am. Chem. Soc., Vol. 104, No. 18, 1982
Eisenstein, Gladysz. et al.
Figure 2. Stereoview of the molecular structure of ac-[(q-C,H,)Re(NO)(PPh3)(=CHC,H,)]'PF6-.CHCI, (ttCHC1,). Table I. Selected Bond Lengths and Bond Angles in ZtCHCl, Figure 1. Calculated Etotal for [(q-C,H5)Re(NO)(PH,)(=CH2)] =CHI ligand is rotated.
as the
(c) Benzylidenes 2k and 2t undergo stereoselective and stereospecific attack, respectively, when treated with a variety of nucleophiles. We then describe the use of physical and theoretical methods (two X-ray crystal structures; extended Huckel MO calculations) to precisely define the stereochemical relationships between the starting materials and products in a-c and to provide insight into the origins of stereocontrol. Results 1. Synthesis and Structural Characterization of Benzylidene Complexes. The benzyl complex (s-C5H5)Re(NO)(PPh3)(CH2CsH5) (1) was treated with 1.1 equiv of Ph3C+PF6- in CD2C12at -78 'C. Proton N M R monitoring (-70 "C) indicated the clean and immediate formation of a cationic benzylidene complex (2k; Scheme 1),lo as evidenced by a characteristic" low-field Re=CH(C6H5) resonance (6 16.08 (s, 1 H)) and a deshielded C5H5resonance (6 5.89 (s, 5 H)). When this solution was warmed to 10-25 "C, 2k began to disappear as a new benzylidene complex, 2t, formed (6 15.30 (s, 1 H), 6.06 (s, 5 H)). At room temperature, the equilibrium ratio of 2t/2k was >99:1. After solvent removal and recrystallization, 2t was isolated in 73-75% yields as yellow, air-stable powder or crystals (Experimental Section) which were stable to >200 "C. Attempts to isolate 2k by low-temperature crystallization gave only impure oils. Additional experiments indicated 2k and 2t to be geometric isomers differing in the orientation of ligands about the Re=C double bond. First, both 2k12 and 2t reacted cleanly with Li(C2H5)3BHto give 1. This transformation established the benzylidene carbon to be electrophilic. Second, irradiation of CD2C12, CD,CN, or (CD3)2C0 solutions of 2t (Hanovia 450-W lamp, through Pyrex) between -78 and -20 "C afforded, after 3 h, clean (55 f 3):(45 f 3) photostationary states of 2t and 2k, as determined by 'H N M R integration of the C5H5resonances. These solutions were allowed to return to thermal equilibrium in the dark. A sample to which p-di-tert-butylbenzene had been added indicated >95% of the original 2t to be present. Thus 2t and 2k can be photointerconverted analogously to C=C double-bond geometric i ~ o m e r s . ~ J ~ (10) For the conversion of planar representations of rhenium complexes into three dimensional structures, we employ the convention shown in Scheme I for alkylidenes* and the following convention for alkyls:
atoms
dist, A
atoms
angle, deg
Re-C1 Re-P1 Re-N Re-C,HSa N-01 C1-H1 c1421
1.949 (6) 2.427 (2) 1.761 (5) 2.333 1.195 (7) 0.98b 1.454 (9)
Re-C1421 Re-Cl-H1 N-Re-C1 P1-Re-N P1-Re41 0 1-N-Re
136.2 (5) 117b 99.8 (2) 91.0 (2) 93.2 (2) 172.7 (5)
a Average distance from Re to C,H, carbons. refined.
H1 was not
Since geometric isomerism due to metal-carbon multiple bonding was without precedent, we turned to an MO analysis for additional insight. Extended Huckel MO calculations have been previously shown to accurately predict metal-alkylidene bonding geometries.I4 Computations on the parent model system15 [(sC5H5)Re(NO)(PH3)(=CH2)]+showed two degenerate energy minima, separated by a 19-kcal barrier, as the rhenium-methylidene bond was rotated through 360' (Figure 1). In the favored conformer, the =CH2 plane eclipsed the (C)Re-N-0 plane. Thus two nondegenerate minima, I and I1 (Scheme I; Newman projections down the benzylidene-rhenium bond) are expected as the benzylidene ligand in 2 is rotated through 360". Space-filling molecular models indicated I1 to be considerably less congested than I. In the former, the benzylidene phenyl ring eclipses the small N O ligand, whereas in the latter it is forced to reside between the bulky PPh3 and medium-sized C5H5ligand. Calculations on the model system [(ll-C5H5)Re(NO)(PH3)(= CHC6H5)]+indicated I1 to be more stable than I as long as the benzylidene phenyl ring was canted at least 25' with respect to the (C)Re-N-0 plane (Le., slightly out of conjugation with Re=C). This relieves a repulsive nonbonding interaction between the N O and an ortho proton of the benzylidene phenyl ring. A definitive structural assignment for 2t was made by X-ray crystallography. A suitable single crystal (0.19 X 0.20 X 0.30 mm; CHC13 monosolvate) was obtained by slow diffusion of petroleum ether (bp 30-60 "C) into a CHC1, solution of 2t. X-ray data were obtained at -158 "C by using monochromated Mo K a (0.71069 A) radiation on a Syntex P i automatic diffractometer. The general techniques employed have been previously described.I6 The unit cell was found to be triclinic, space group Pi ( Z = 2), with lattice parameters a = 8.984 (5) A, b = 14.008 (6) A, c = 13.965 (7) A, a = 109.06 (4)', p = 101.92 (4)', y = 75.99 (4)'. Of 5666 reflections with 28 < 50' collected, 5070 with I 1 3 4 I ) were used in the final refinement. The positions of the Re and P(1) atoms were obtained from a three-dimensional Patterson map. Several least-squares refinements yielded all non-hydrogen atoms, including the CHC13,in the unit cell. Absorption corrections were then applied after which all hydrogens were located from a difference Fourier map. The rhenium, chlorine, fluorine, and
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I (11) Brookhart, M.; Nelson, G. 0. J . Am. Chem. Sot. 1977,99, 6099. (12) Unless otherwise noted, reactions of 2k were conducted with material prepared in situ at -78 "C. (13) (a) Saltiel, J.; DAgostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, 0. C. In "Organic Photochemistry"; Chapman, 0. L., Ed.; Marcel Dekker: New York, 1973; Vol. 3, pp 1-1 13. (b) Saltiel, J.; Charlton, J. L. In "Rearrangements in Ground Excited States"; de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, p 25.
(14) (a) Schilling, B. E. R.; Hoffmann, R.; Faller, J. W. J . Am. Chem. Soc. 1979,101, 592. (b) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. Ibid. 1979,101, 585. (15) (a) The isolation of the methylidene complex [(?-C,H,)Re(NO)(PPh3)(=CH2)]+PF6- has been described in a separate p~blication.'~~ (b) Tam, W.; Lin, G.-Y.; Wong, W. K.; Kid, W. A,; Wong, V. K.; Gladysz, J. A. J . Am. Chem. Sot. 1982,104, 141. (16) Strouse, J.; Layten, S. W.; Strouse, C. E. J . Am. Chem. Sot. 1977, 99, 562.
Benzylidine
J . Am. Chem. SOC.,Vol. 104, No. 18, 1982 4867
[(v- C,Hs)R e ( N 0 )(PPhJ (=CHC6Hs)]+PF6-
Table 11. Rate Constants for the Re=C Bond Rotation 2k + 2t in CD,Cl, a entry
temp ( k 0 . 1 "C)
1 2 3 4 5b 6 7 8 9
29.5 24.0 19.0 19.0 19.0 14.0 10.0 10.0
1o5kObs,,
20,
s-l
69.2 f 0.6 37.6 t 0.6 20.5 0.2 19.2 0.2 20.9 f 0.3 11.5 f 0.2 4.81 k 0.10 6.93 0.10 2.61 0.03
* *
* *
4.0
a [2k], = 0.038 - 0.042 M. Entries 1-5 were followed through >2t,,,; others were followed through a t least one t,,,. See Experimental Section for further details. 2k photochemically generated at -78 O C .
phosphorus atoms were refined with anisotropic temperature factors, and all other non-hydrogen atoms were refined with isotropic thermal parameters. All hydrogen atoms were held at positions indicated from the map with assigned isotropic thermal parameters. The final R index was 0.039 with R , = 0.049.'' The molecular structure of 2tCHCl3 thus obtained is shown in Figure 2. The near eclipsing of the (C1)Re-N-01 and H1Cl-C21 planes (torsion angle = 4.0 f 0.7') is evident. Thus 2t has the structure approximated by Newman projection I1 (Scheme I). Since the M O calculations indicate only two energy minima as rhenium-alkylidene bonds are rotated through 360', 2k can confidently be assigned the structure I. An exhaustive list of bond distances and angles in 2tCHC1, and a crystal packing diagram are provided in the supplementary material. Important bond distances and angles are summarized in Table I. The P1-Re-N angle of 91.0 (2)' is expected based upon crystal structures of other, formally octahedral, (7-C5H5)ML3 (M = Mn, Re) complexes.lSa Hence the corresponding angles in Newman projections I-XIV (normally 120') have been compressed. As anticipated by the calculations, the least-squares plane of the benzylidene phenyl ring (Figure 2) is canted 19.9 f 0 . 8 O with respect to the Re=C vector. Benzylidene complexes 2k and 2t were further characterized by isomerization rate measurements and UV-visible spectroscopy. The first-order rate constant for equilibration of a (55 f 3):(45 f 3) 2t/2k photostationary state (in CD2C12) back to 2t was measured by IH N M R at 19 'C (entry 5, Table 11). The kobsd obtained was in good agreement with the kobsddetermined (19 'C) from 2k prepared in situ from (&H5)Re(NO)(PPh3)(CH2C6H5)and Ph3C+PF6-(entries 3 and 4, Table 11). Hence, data obtained by the latter route can reliably be used to calculate activation parameters for rotation about rhenium-alkylidene bonds. Rate constants for 2k 2t were measured from 4 'C (tl,2 = 443 min) to 29.5 'C (t112 = 17 min), as summarized in Table 11. These yielded AH* = 20.9 f 0.4 kcal/mol and A S * = -3.8 f 0.2 eu.lSb The absorption spectrum of 2t displayed a, , ,A at 365 nm (e 13000) in CH3CN. A series of spectra (Figure 3) were recorded as a CH2C122t/2k photostationary state was warmed from -78 'C to room temperature. An isosbestic point was evident as 2k isomerized to 2t. Its location indicates that, , ,A for 2k is at a shorter wavelength than for 2t. 2. Stereochemistry of Nucleophilic Attack at Benzylidene Carbon. The exclusive formation of the less stable geometric
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(17) All least-squares refinements computed the agreement factors R and R , according to R = ZllFal - ~ F c ~ ~ and / ~ R,~ F= o[xwillFoI ~ - lFc112/ xwilF0I']'/', where Fo and F, are the observed and calculated structure factors,
respectively, and will2= l/u(Fo). The parameter minimized in all leastsquares refinements was xwillFol - lFc112, (18) (a) Caulton, K. G. Coord. Chem. Rev. 1981, 38, 1. (b) A reviewer has raised the possibility of PPh3 dissociation during the 2k 2t isomerization. Free phosphines complex (with varying Kq)to the benzylidene carbons of 2k and 2t, as shown with PMel below. Thus isomerization rates comparable to those in Table I cannot be obtained in the presence of added PPh3. However, when 0.5 equiv of P(p-C&CH,), is added to Zt at 25 OC, no [(?C5Hs)Re(No)(P(p-C6H,CH,),)(=cHc6H5)]+PF6- (independently synthesized analogously to 2t) forms.
-
00'2%
300
3w
400 WAVELENGTH (nml
450
5w
Figure 3. Absorption spectra recorded as a (55 f 3):(45 f 3) 2t/2k mixture in CH2C12(prepared by photolysis at -78 " C ) was warmed to room temperature: initial spectrum, trace a; final spectrum, trace g.
isomer 2k upon reaction of 1 with Ph3C+PF6-suggested to us that only one of the two diastereotopic a-hydrides underwent abstraction. The widely separated a-hydride ' H N M R chemical shifts, 6 3.50 and 2.89 (CDC1,; couplings: Table V), also hinted at the possibility of differential reactivity. Testing of this hypothesis required the synthesis of 1 with an a-deuterium label at either the pro-R or pro-S site. Obvious routes to such a substrate did not exist, so exploratory reactions of 2k and 2t with nucleophiles were conducted. Benzylidene 2t was reacted with Li(C2H5),BD. Subsequently isolated was l-a-dl-t (92% yield), a compound with two chiral centers (see step a, Scheme 111). The 'H N M R spectrum of l-a-d,-t in CD2C12(6 3.41 (br d, JlH-+= 8 Hz, J I ~I-2 Hz)) ~ ~ indicated that one of the two diastereotopic hydrogens normally present in 1 was completely absent. Hence the addition of D- to 2t occurred stereospecifically on one benzylidene face. As little as 1% of the other possible diastereomer would have been detected. Authentic samples of the other 1 - a d l diastereomer, l-a-dl-k, were available by reaction of Li(C2H5),BD with 2k.I' Reaction in this case was stereoselectiue; a (92 f 1):(8 A 1) mixture of l-a-d,-k/l-a-d,-t was obtained. The lH NMR monitored reaction of 2k with Li(C2H5),BD was complete within 3 min at -73 'C. Since the rate constants in Table I1 indicate that no 2k 2t isomerization occurs on this time scale, the lack of stereospecificity must be ascribed to a small amount of Li(C2H5),BD attack upon a second benzylidene face of 2k. The possibility of crystallographically determining the direction of Li(C2H5),BD attack upon 2t was considered. However, a neutron diffraction structure is required to distinguish H from D, and sufficiently large crystals of 1-a-d,-t could not be grown. Fortunately, 2t underwent similar stereospecific attack (as assayed by IH N M R spectroscopy of the reaction mixtures prior to any crystallizations) with nucleophiles CH,Li, CH,CH,MgBr, C6H5CH2MgC1,PMe,, and CH,ONa. Diastereomerically pure adducts (~-C5H5)Re(NO)(PPh3)(CH(CH3)C6H5) (3t), (7C5H5)Re(N0)(PPh3)(CH(CH2CH3)C6H5) (4th (TCSH,)R~(No)(PPh3)(CH(CH2C6H5)C6H5) (5tL [(o-C5HdRe(NO)(PPh3)(CH(PMe3)C6H5)]+PF6(6t), and (&H5)Re(NO)(PPh3)(CH(OCH3)C6H5)(7t) were subsequently isolated in 75-90% yields. Benzylidene isomer 2k was stereoselectively attacked by these nucleophiles.12 The predominantly formed diastereomer (3k-7k) was in each case opposite to the one obtained from 2t. Diastereomer ratios were assayed by 'H N M R spectroscopy in situ and/or prior to any workup crystallizations: 3k/3t, (94 f 1):(6 1); 4k/4t, (95 f 1):(5 f 1); 5k/5t, (94 f 1):(6 f 1); 6k/6t, (94 f 1):(6 f 1); 7k/7t, (78-85):(22-15). Complexes 3k-7k could be obtained in pure form by fractional crystallization. Spectroscopic properties of 3-7 are summarized in Table 111. The preceding data constitute compelling evidence that Li(C2HS),BD,CH3Li, CH3CH2MgBr,C6H5CH2MgCl,PMe,, and CH,ONa each attack 2t from the same direction. The effective bulk of Li(C2H5),BD should be considerably greater than that of an isolated D-. Thus it seems highly unlikely that Li(C2H5),BD would stereospecifically attack one benzylidene face of 2t (see projection 11, Scheme I), while R-, Me3P,and CH30- nucleophiles stereospecifically attack the other. Furthermore, the dominant
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*
4868
J . Am. Chem. SOC.,Vol. 104, No. 18, 1982
Eisenstein, Gladysz. et al.
9
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J . A m . Chem. SOC.,Vol. 104, No. 18, 1982 4869
Benzylidine [(q- CsHs)Re(NO)(PPh3)(=CHC6H5)]+PF6-
m I1
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4870 J. Am. Chem. SOC., Vol. 104, No. 18, 1982
Eisenstein, Gladysz, et al.
Figure 4. Stereoview of the molecular structure of (ss,RR)-(?-C,H,)Re(NO)(PPh3)(CH(CH2C~H5)C~H5).CH2Cl~ (5t.CH2C12).
Scheme 11. Assignment of the Direction of Nucleophilic Attack upon 2t
production of opposite diastereomers from 2k requires that these nucleophiles approach I and I1 from the same “directi~n”.’~Thus an X-ray structure determination of any of 3t-7t or 3k-7k allows configurations to be assigned to all diastereomers. Suitable crystals of 3t could not be grown, but single crystals of 5t (as a CHzClz monosolvate) were obtained by layering a CHZCl2solution with hexane. Since these crystals lost CH2C12 upon standing at room temperature, the one selected for X-ray analysis (0.20 X 0.25 X 0.40 mm) was isolated and mounted immediately prior to data collection (-158 ‘C, Mo K a irradiation as described above). The unit cell was found to be monoclinic, space group P2Jc (Z= 4), with lattice arameters a = 12.027 (4) A, b = 16.083 (6) A, c = 18.446 (6) /3 = 110.57 (2)’. Of 6441 reflections with 20 < 50’ collected, 5309 with Z I 3 4 were used in the final refinement. Positions of all atoms, including the solvate CH2Cl2,were located as outlined for 2t.CHCl3 above. The rhenium, chlorine, and phosphorus atoms were refined with anisotropic temperature factors, and all other non-hydrogen atoms were refined with isotropic thermal parameters. Hydrogen atoms were held at positions indicated from the difference Fourier map with assigned thermal parameters. The final R index was 0.025 with R , = 0.038.‘’ The molecular structure of 5t.CH2ClZthus obtained is shown in Figure 4. The enantiomer of the structure in Figure 4 is of course present in the unit cell as well. Absolute configurations at the chiral centers in 5t are therefore (Re, C) S,S (Figure 4) and R,R. The configuration of 5t indicates that it is formed by C&CHzMgCl attack upon the si-benzylidene face, antiperiplanar to the PPh3, as shown in Scheme 11. The initially generated conformer, 111, is however not the one found in the crystal. Rather, a 120’ Re-C rotation occurs which orients the -CH2C6H5 substituent between the C5H5and PPh3 ligands (IV, Scheme 11). By
w,
(19) We define ‘direction” or “side” with reference to the rhenium ligands in I and 11. Nucleophilic attack upon the benzylidene carbon from a direction anti to the PPh, would constitute re face attack in I but si face attack in 11.
Table IV. Selected Bond Lengths and Bond Angles in StCH,Cl, atoms dist, A atoms angle, deg Re-C1 2.215 (4) Re41422 115.7 (2) Re-P1 2.337 (1) R e 41 4 2 1 107.8 (3) Re-N1 1.747 (3) 114.0 (3) Clc2-C31 Re-C,HSa 2.301 C2ClC21 114.7 (3) N1-01 1.216 (4) N 1-Re-C 1 90.6 (1) c1c2 1.539 (5) P1-Re-N1 90.0 (1) P1-Re41 Cl-C21 1.502 (5) 93.6 (1) 01-N1-Re C2431 1.512 (5) 178.3 (3) Average distance from Re t o C,H, carbons. assumption of an identical direction19 of attack for the other nucleophiles (vide supra), configurations are assigned to the chiral centers in l-a-d,-t, l-a-d,-k, 3t-7t, and 3k-7k as summarized in Table 111. An exhaustive list of bond distances and angles in 5t.CHZClz and a crystal packing diagram are provided in the supplementary material. Important bond distances and angles are summarized in Table IV. The R e 4 1 4 2 1 plane was found to make a 92.7 f 0.5’ angle with the least-squares plane of the Re-CHRC6H5 phenyl ring. 3. Stereochemistry of Hydride Abstraction from (7-C5H5)Re(NO)(PPh3)(CH2C6HS).With the absolute configurations of the enantiomers comprising l-a-dl-t established, the stereochemistry of the reaction of l-a-dl-t with Ph3C+PF6-could be definitively probed. Proton N M R monitoring at -70 ‘C (Scheme 111, step b) indicated the exclusive formation of H- abstraction product 2k-a-dl and then, upon warming (step b’), 2 t - a d , . Byproduct Ph,CH was isolated. Its mass spectrum indicated Ph3CD to be present at natural abundance level. To ensure that the unusual stereospecificity of step b was not due to an extraordinary kinetic isotope effect, a closed stereochemical cycle was constructed. Thus, isolated 2t-adl was treated with Li(C2H5)3BH(Scheme 111, step c); l-a-dl-k formed stereospecifically. As little as 1% of l-a-dl-t would have been detected. These results evidence a remarkable degree of metal-mediated stereocontrol over the first three steps (a, b, b’, c) of the cycle. The 1-a-d,-k from step c (Scheme 111) was treated with Ph3C+PF6- (step d). Proton N M R monitoring indicated the formation of 2k (D- abstraction product) and then, upon warming (step d’), 2t. However, some Ph3CH was evident by ‘H NMR. Careful integration of Re=C&H5 and C5H5resonances (both in situ and after isolation) indicated the product from step d’ to be a (92 f 2):(8 f 2) 2t-a-do/2t-a-dl mixture. The triphenylmethane by product was isolated; mass spectral analysis indicated a (89 f 1):(11 f 1) Ph3CD/Ph3CH mixture.20 Thus, on the order of 8-10% H- abstraction occurs in step d. The unusual features of Scheme I11 are more evident when viewed in Newman projection form (Scheme IV). In advance of any experiment, it would not have been surprising if D- abstraction from l-a-d,-t (V) had occurred (i.e., a “quasi-microscopic reverse” of D- attack). Instead, H- loss occurs, corresponding to (20) The Ph,CD/Ph,CH ratio may underestimate stereoselectivity in reactions involving predominant deuterium abstraction. For instance, we find the triphenylmethane generated by reaction of Ph3C+PF6with (q-C5HS)Re(NO)(PPh3)(CD3)to be a (96 f 1):(4 1) Ph,CD/Ph,CH mixture. We attribute part of the Ph$H to side reactions involving nondeuterated ligands and/or adventitious H- sources.
*
-
J. Am. Chem. SOC.,Vol. 104, No. 18, 1982 4871
Benzylidine [(q CsHs)R e ( N 0 )(PPhJ (=CHc6Hj)]+PF6-
Scheme IV. Newman Projections of Species Involved in Scheme 111, Including All 1-a-d, Rotamers
Scheme 111. An Organometallic Walden-Type Cycle (“Long” Cycle)
3 2I
f2 11
Li (C2H5 ( a ) ) 3 ED
Re’
ON’
ON’
‘PPh3
I
,-
‘PPh3
-I - a - d , - t
ZL (d’)
Re
c6H5J%:3
0 6‘ 5’
10-25’‘
ON BH P P h s
H
ON
0
/‘\ ‘€iH5
92:8 stereoselectivity
PF;
/c\
‘gH5
D
PF;
1
(b’) 10-25 “C
H/i\D ‘gH
5
‘€iH5
11
I1
PF;
abstraction of the pro-R a-hydride from (q-CSHs)Re(NO)(PPh3)(CH2C6HS)(1). Possible rationales for this reactivity will be presented in the Discussion. A “short” cycle, interconverting 2k and 1-a-d,-k as shown in Scheme V, is implicit in the reactions described above. The simplicity of Scheme V vis-&vis the complexity of Scheme IV can be explained as follows, without reference to mechanism. When the cycle is started with 2k (Scheme V), D- is delivered into what would be the pro-R position of 1. Hence D- is abstracted in the subsequent step with Ph3C+PF[. However, when the cycle is started with 2t (Scheme IV), D- is delivered into what would be the pro-S position of 1. Hence H- is abstracted in the subsequent step with Ph&+PF6-. 4. Additional Experiments Related to Reaction Stereocontrol. Further experiments were conducted to provide insight into Schemes I-V. First, the a-proton . h p l H in the ‘H N M R spectrum of 1 in CDCI3 were examined as a function of temperature. The data shown in Table V were obtained. Couplings measured in CD2C12were similar. As will be described in the Discussion, hIp-iHa values have been previously correlated to conformer populations (e.g ., Va-c) in (q-C,H,) Fe( CO) (PX3)(CH2R) systems.21 (21) Stanley, K.; Baird, M. C. J . Am. Chem. SOC.1975, 97,4292.
!G k
3h% ON :‘
‘gH5
‘6 ONH5 B P P h 3
! !K Scheme V.
A “Short” Cycle -70T L\(C2H5)360
a’”””
stereoselectivi t y
92:8
ON
H
I
D : H stereoselectivity
92:8 -78 ‘C
-.--- -
(L-a-di
-k)
Second, the dissymmetry of the Hiickel orbitals with respect to the Re=CHR plane (which could provide an electronic basis for the nonequivalent reactivities of the two diastereotopic al-
4872 J . Am. Chem. SOC.,Vol. 104, No. 18, 1982
Eisensrein, Gladysz, et al.
Table V. Variation of 31P-1Hand ‘HJH NMR Coupling Constants ( H z ) ~in (q-C,H,)Re(NO)(PPh,)(CH,C,H,) with Temperature“ temp, K 320 310 300 290 280 270 260 25 0 240
J31p-1H,
J32p-1Ha!
JIHa-lHol’
3.16 3.10 3.04 2.92 2.83 2.75 2.60 2.32 2.26
11.70 11.70 11.72 11.72 11.73 11.72 11.75 11.70 11.72
7.64 7.74 7.86 7.88 7.81 7.92 7.90 7.82 7.78
I
401
c
1
80
1201 I I
160
2bO
I2hO
280
1
820
360
I
” In CDCI,; data are the average of two runs. Spectra were recorded at 0.061 Hz/data point. Lowtemperature coupling constants are less accurate than the others due to peak broadening. Figure 6. Calculated change in C-H’ bond order and electron density on H’ as the R e C u bond in (&,H,)Re(NO)(PH,)(CH,H’) is rotated through 360’ (C-H’ = bond arbitrarily elongated to 1.4 A).
a) 0 I
Scheme VI. “Short” Cycles Involving CH30Addition/Abstraction
e
(i)
b)
other rofamers
O N m P F h 3
‘sH5
\
Ph3C’PF;
ON/*-rPPh3 ‘€iH5
/
stereospecific
--I1
--I Xa-
(ey P
Figure 5. LUMO of [(T~C,H,)R~(NO)(PH,)(=CH~)]*: (a) schematic; (b) contour diagram (for 0 = f0.20, 10.15, f O . l O , fO.05, fO.01) in P - R e 4 plane.
(22) Trost, B. M.; Salzmann, T. N. J. Am. Chem. SOC.1973, 95, 6840. See footnote 12.
NaOCH3 78-85% stereoselective 5rN$;\.
’ 3 :&
other rotamers
(11)
ON
kylidene faces) was examined. In the model compound [(qCsHs)Re(N0)(PH3)(=CH2)]+, the LUMO was found to be the d-p T antibonding combination. This orbital should command the major interaction with the incoming nucleophile, but as shown in Figure 5, it is symmetrical with respect to the Re=CH2 plane. A model nucleophile, H-, was placed 2 A from the methylidene The total energy indicated in [(q-C,Hs)Re(NO)(PH3)(=CH2)]+. a slight preference for an approach antiperiplanar (over synperiplanar) to the PH,. Third, properties of the model alkyl (q-CsH5)Re(NO)(PH3)(CH3) were also probed via extended Hiickel MO calculations. The barrier to rotation about the Re-CH, bond was found to be -3 kcal/mol, with a staggered conformation (hydrogen antiperiplanar to C5H5) as energy minimum. As the Re-CH, bond was rotated, no particular weakening (bond-order decrease) of any C-H bond was noted. One C-H bond of the Re-CH3 group (C-H’) was elongated from 1.09 to 1.40 A in order to more closely resemble a hydride abstraction transition state. Now as the Re-CH2H’ bond was rotated, a significant weakening of the C-H’ bond (and increase of electron density on H’) occurred when it was antiperiplanar to the PH, ligand (see Figure 6). Finally, reactions of 7t and 7k with Ph3C+PF6-were also investigated.* When 7t was treated with Ph3C+PF6-at -78 “cin a ‘H N M R monitored reaction, 2t was observed to form exclusively. Thus CH,O- is chemospecificallyZ2and stereospecifically abstracted. Coupled with the reaction of 2t and NaOCH, described above, this reaction completes a second “short cycle”, as shown in Scheme VI. Similarly, when a 90:lO 7k/7t mixture was treated with Ph3C+PF6-at -78 OC, a 9O:lO 2k/2t mixture
(711
\
H I ($I$
Ph3C’PFE stereospecific
/
H
XI! (i,Ll
formed. Thus C H 3 0 - is stereospecifically abstracted from 7k, closing the other short cycle in Scheme VI. These data require that -OCH3 attack and -OCH3 abstraction occur from the same “side”lg of the complex. Discussion 1. Benzylidene Complexes: Structure and Bonding. To our knowledge, the existence of geometric isomers due to metal-carbon multiple bonding is without precedent. Commonly used terms for distinguishing geometric isomers (cis/trans; Z / E ) are clearly inadequate for 2k and 2t. According to IUPAC n ~ m e n c l a t u r e , ~ ~ I (2k) and I1 ( 2 ) are unambiguously designated as synclinal (sc) and anticlinal ( a c ) , respectively. Absolute configurations at rhenium are assigned according to the Baird-Sloan modification of the Cahn-Ingold-Prelog priority rules.24 The C5H5ligand is considered to be a pseudoatom of atomic number 30. Hence all figures in this paper have have an S-rhenium configuration. As has been observed with other metals and unsaturated liga n d ~extended , ~ ~ Hiickel calculations accurately predict rhenium alkylidene bonding geometries. The optimal alkylidene position ( 2 3 ) Pure Appl. Chem. 1976,45, 11. See section E-5.6, p 24. A synclinal conformer of 2 is one in which the highest priorityz4groups on Re(C,H,) and C(C,H,) define a 60 =k 30” torsion angle. An anticlinal conformer IS one in which the highest priority groups define a 120 k 30° torsion angle. (24) (a) Stanley, K.; Baird, M. C. J . Am. Chem. SOC.1975, 97, 6598. (b) Sloan, T. Top. Stereochem. 1981, 12, 1 .
Benzyiidine ((7 - C,H,) R e ( N 0 )(PPh3)(=CHC6H5)]+PF; is obtained when the empty p orbital of the carbene fragment (counted as neutral) maximally overlaps the highest occupied d orbital of the metal fragment. In a d6 metal fragment such as (q-CsHS)Mn(C0)2,which has a plane of symmetry, the highest occupied d orbital would be in a horizontal plane in the Newman projections perspectives utilized in this paper.I4 Accordingly, the X-ray crystal structure of (q-C,H,)Mn(CO)2(=C(CH3)2) (Mn=C distance = 1.868 A) shows that the alkylidene plane bisects the CSH, ring.25 In (q-C5Hs)Re(NO)(PPh,)+(and related isoelectronic metal fragments), no symmetry is maintained and the highest occupied d orbital rotates into a plane containing the Re-P bond (compare to Figure 5),14 resulting in alkylidene geometries I and 11. The crystal structures of three benzylidene complexes, ( q CSH5)2Ta(CH2C6H,)(=CHc~Hs),26 (q-C5MeS)Ta(CH2C6HS)2(=CHC6H,),27and (q-CSH5)2W=cHC6H5,2shave been previously reported. Since these all contain third-row metals, their bonding features may be closely compared to 2t. For the first and third complexes, M=C,+, bond angles are reasonably close to those expected for sp2 carbons (1 35, 133O), and M=C, bond lengths are 2.07 and 2.05 A, respectively. However, (qC,MeS)Ta(CH2C6H5)2(~Hc6Hs) is a 14-electron complex, so the benzylidene ligand adopts a "T" shaped29geometry (LTa= = 160°, with a proposed LTa=C,-H of ca. 90') to C,-Cip, help relieve the electron deficiency. A short Ta=C, distance of 1.88 A was also noted. Benzylidene 2t is an 18-electron complex, and accordingly its LRe=C,-Ci, of 136O more closely resembles the former class of complexes. The +Re=C, distance in 2t, 1.949 (6) A, is intermediate between the two types of complexes. The bond contraction vis-5-vis (q-CSH5)2W=CHC6HS may be due to the positive charge on rhenium. Together with the X-ray structure of the formyl (q-CSH,)Re(NO)(PPh,)(CHO) (S),,O this study marks the first time that accurate metal-carbon bond lengths have been determined for a homologous series of alkyl, formyl, and alkylidene c ~ m p l e x e s . ~ ~ As shown below, the Re-C, distances decrease as the r-acceptor capabilities of the ligands increase. The R e C , distance in formyl 8 is closer to that of benzylidene 2t than it is to alkyl St. This supports our previous c o n c l u ~ i o that n ~ ~a ~dipolar ~ ~ ~ alkylidene-like resonance form +Re=C(H)O- makes a substantial contribution to the ground state of 8.
9'-
2 215 (4)
Re
I-
i
2 0 5 5 (10) E\
/c\H CBH5CHZ
1
8
C6H5
5t t
Re
11-
C6H5
1949 16)
/c\H 2t
-
The activation parameters for 2k 2t isomerization contain a component arising from steric interactions. In view of recent data on homologous vinylidene complexes [ (q-CSHS)Re(NO)(25) Friedrich, P.; Bed, G.; Fischer, E. 0.; Huttner, G. J . Organomet. Chem. 1977. 139. C68. (26) Schiock, RT R.; Messerle, L. W.; Wood, C . D.; Guggenberger, L. J. J . Am. Chem. SOC.1978, 100, 3793. (27) Messerle, L. W.; Jennische, P.; Schrock, R. R.; Stucky, G. J . Am. Chem. SOC.1980, 102, 6144. (28) Marsella, J. A.; Folting, K.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. SOC.1981, 103, 5596. (29) Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J . Am. Chem. SOC. 1980, 102, 7667. (30) Wong, W.-K.; Tam, W.; Strouse, C . E.: Gladysz, J. A. J . Chem. Soc., Chem. Commun. 1979, 530. (31) (a) An X-ray crystal structure of optically pure (-)-R-(q-C,H5)Re(NO)(PPh,)(CH,C,HS) has also been completed and will be described in a separate publication."b The Re95% of the original Zt to be present. Rates of Isomerization: Zk 2t. To a septum-capped N M R tube was added 0.0148-0.0168 g of (q-C5H,)Re(NO)(PPh,)(cH2c6H5) (1) in 0.300 mL of CD2C12. The N M R tube was then cooled to the temperature of data collection (Table 11), whereupon 1.2 equiv of Ph$+PF6in 0.200 mL of CD2C12was added via gas-tight syringe. The N M R tube was then quickly transferred to a N M R probe that had been preequilibrated to the appropriate temperature. Disappearance of 2k was monitored by integration of the b 16.08 resonance. Standard In (integral) vs. time plots yielded the data in Table 11. AH*and AS*were obtained from In (kobsd/7') vs. 1 / T plots. Preparation of (S,RR)-(q-C,H,)Re(NO) (PPh,) (CHDC6H5) (1-adi-t). Benzylidene 2t (0.154 g, 0.198 mmol) was dissolved in 20 mL of CH2C12in a 100-mL Schlenk flask and cooled to -78 OC. Then 0.220 mL (0.220 mmol) of Li(C2H5),BD (1.0 M in THF) was added dropwise. The solution was allowed to warm to room temperature and the solvent removed under oil pump vacuum. The resulting orange residue was extracted with benzene and filtered through a 2-in. silica gel plug. The benzene was removed and the residue recrystallized from CH2C12/hexanes to give l-a-dl-tas an orange solid (0.116 g, 0.183 mmol, 92%), mp 220-222 OC dec. Spectral data: Tables I11 and VI. Preparation of (SR ,RS)-(7-C,H,)Re(NO) (PPh,) (CHDC6H5)(1-adl-k). A. To a -78 OC solution of 2k (prepared from 0.187 mmol of 1 as described above) was added dropwise 0.280 mL of Li(C2H5),BD (1.0 M in THF). After 15 min at -78 OC, the reaction was allowed to warm to room temperature while solvent was simultaneously removed under oil
-
J . Am. Chem. SOC.,Vol. 104, No. 18, 1982 4811
Benzylidine [ ( q - C5H5)Re(NO)(PPh3)(=CHC6H5)]+PF{
mmol, 90%) of 5t as an orange powder, mu 222-225 "C dec. SDectroscopic data are given in Tabies'III and Vi. ions, m / e for '"Re (% of base peak) Preparation of ( SR ,RS) - [ (r) -C,H ) Re ( NO) ( P P h, ) ( CHcomM+ M+-R M+-PPh3 PPh3+ other (PM~&Hs)]+PF,- (6t). Benzylidene 2t (0.116 g, 0.149 mmol) was plex dissolved in 20 mL of CH2C12in a Schlenk flask and cooled to -78 OC. _.. 5 44 LbL 3 14 l - a d , - t 636 Then 0.030 mL (0.022 g, 0295 mmol) of PMe, was added. The resulting (41.5) (20.9) (47.8) (1W orange solution was stirred for 5 min at -78 OC. The reaction was then 262 5 44 374 l-ad,-k 636 allowed to warm to room temperature while solvent was simultaneously (27.3) (62.3) (40.4) (100) removed under oil pump vacuum.52 The resulting solid was taken up 544 35 9 262 387 3t 649 in CH2C12;hexanes were then layered onto this solution. On standing, (13.8) (25.8) (73.5) (100) (6.4) orange needles of 6t formed. These were collected by filtration and dried 544 359 262 387 3k 649 (0.105 g, 0.123 mmol, 82%), mp 229-230 OC dec. Spectroscopic data (11.5) (29.2) (72.1) (5.7) (100) are given in Table 111. 544 35 9 262 401 4t 663 Preparation of (SR,RS)-(?-CsHs)Re(NO)(PPL,)(CH(OCH3)C6H,) (14.4) (21.4) (82.1) (100) (6.8) (7t). To a solution of 1 (0.385 g, 0.609 mmol) in dry CH2CI2(30 mL) 359 544 262 401 4k 663 at -78 OC was added solid Ph,CCPF; (0.334 g, 0.861 mmol). The (27.1) (12.1) (100) (6.6) (9.2) resulting mixture was stirred for 30 min and then allowed to warm to 634b 544 262 463 5t not room temperature. After 4 h at room temperature, a CH,ONa solution observeda (28.7) (29.9) (1.2) (100) prepared from N a (0.64 g, 27.8 mmol) and anhydrous CH,OH (1 5 mL) 359 was added. The resulting mixture was stirred for 20 min, whereupon the (40.5) solvent was removed under reduced pressure. The residue was extracted 634b 544 5k 7 25 463 262 with CH2C1?' and recrystallized from CH2C12/hexanes to give yellow (51.1) (28.1) (6.5) (3.9) (100) crystals of 7t (0.299 g, 0.452 mmol, 74%), mp 169-172 OC dec. Spec359 troscopic data are given in Tables 111 and VI. Anal. Calcd for (29.5) C31H29N02PRe:C, 56.01; H , 4.40; N , 2.11; P, 4.66. Found: C, 55.81; 635c 544 not 262 7t 665 H , 4.47; N , 2.34; P, 4.61. (21.7) observed (81.1) (1.1) (100) Preparation of (SR,RS)-(q-C5Hs)Re(NO)(PPh,)(CH(CH3)C6H,) 373 (3k). To a -78 OC solution of 2k (prepared from 0.160 mmol of 1 as (14.3) described above) was added dropwise 0.350 mL of CH3Li (1.4 M in ether). After 5 min at -78 OC (during which time the solution turned a The absence of a M+ ion for 5t but not 5k may be a consefrom yellow to orange), the reaction was allowed to warm to room temquence of the different probe temperatures used: S t , 210 'C; Sk, perature while solvent was simultaneously removed under oil pump 190°C. M+-CH,C,H,. CM+-NO. vacuum. The residue was extracted with benzene/CH2C12 (1:l) and filtered through a 2-in. silica gel plug. The solvent was removed and the pump vacuum. The residue was extracted with benzene and filtered residue chromatographed on a 13 X 2.5 cm silica gel column (1:l through a 2-in. silica gel plug. The benzene was removed by rotary CH2C12/hexanes). The orange band was collected. Solvent removal evaporation and the orange solid IH N M R assayed. Integration of the afforded an orange powder (0.0893 g, 0.138 mmol, 86%), mp 186-188 ReCffDC& resonances indicated a (92 f 2):(8 f 2) ratio of l-a-d,-k OC dec, which was shown by 'H N M R (CSH, resonances) to be a (94 to l-cu-dl-t. Recrystallization from CH2CI2/hexanesgave 0.096 g (0.151 f 1):(6 f 1) 3k/3t mixture.53 Spectroscopic data are given in Tables mmol, 80%) of (predominantly) 1-a-d,-k. 111 and VI. B. Diastereomerically pure l-a-dl-kwas prepared by the reaction of Preparation of (SR,RS)-(q-C,H,)Re(NO)(PPh,)(CHLi(C2H,),BH with ?t-cu-d, (preparation below) in a procedure analogous (CH2CH3)C6H,) (4k). To a -78 OC solution of 2k (prepared from 0.300 to the one above for l-a-d,-t. Spectral data are given in Tables 111 and mmol of 1 as described above) was added dropwise 0.400 mL of CH,CVI. H2MgBr (3.0 M in ether). After 5 min at -78 OC, the reaction was Preparation of (SS,RR)-(q-CSHs)Re(NO)(PPh3)(CH(CH3)C6Hs) allowed to warm to room temperature while solvent was simultaneously (3t). Benzylidene 2t (0.306 g, 0.393 mmol) was dissolved in 40 mL of removed under oil pump vacuum. The residue was extracted with CH2C12in a Schlenk flask and cooled to -78 OC. Then 0.320 mL of benzene and filtered through a 2-in. silica gel plug in a glovebox. The CH,Li (1.4 M in ether) was added dropwise. The solution turned orange benzene was removed under oil pump vacuum, and the resulting residue and was allowed to warm to room temperature. Solvent was removed was chromatographed on a 13 X 2.5 cm silica gel column (1:1 under oil pump vacuum. The resulting residue was extracted with CH2Cl,/hexanes). The orange band was collected. Solvent removal benzene and filtered through a 2-in. silica gel plug. The benzene was afforded an orange powder (0.100 g, 0.150 mmol, SO%), mp 172-175 O C removed by rotary evaporation, and the orange solid that remained was dec, which was shown by 'H N M R (C,HS resonances) to be a (94 f chromatographed on a 13 X 2.5 cm silica gel column (1:l CH2C12/hex1):(6 1) 4k/4t mixture.53 Spectroscopic data are given in Tables I11 anes). The orange band was collected, and solvent was removed under and VI. oil pump vacuum to give 0.214 g (0.330 mmol, 84%) of 3t as an orange Preparation of (SR,RS)-(~-C,H,)Re(NO)(PPh,)(CHpowder, mp 237-240 OC dec. Spectroscopic data are given in Tables I11 (CH~C~H,)C~HS) (5k). To a -78 OC sohtion of 2k (prepared from 0.160 and VI. mmol of 1 as described above) was added dropwise 0.180 mL of C6HSPrep a r at ion of ( SS ,RR ) - ( q - C H ) Re ( N 0 ) ( P P h ) ( CH CHzMgCl (1.8 M in THF). After 15 min at -78 OC, the reaction was (CH2CH3)C6H,)( 4 ) . Benzylidene 2t (0.105 g, 0.135 mmol) was disallowed to warm to room temperature while solvent was simultaneously solved in 20 mL of CH2C12 in a Schlenk flask and cooled to -78 OC. removed under oil pump vacuum. The residue was extracted with Then 0.180 mL of CH3CH2MgBr (3 M in T H F ) was added dropwise. benzene and filtered through a 2-in. silica gel plug in a glovebox. The After 15 min at -78 OC, the reaction was allowed to warm to room benzene was removed under oil pump vacuum, and the resulting oil was temperature while solvent was simultaneously removed under oil pump chromatographed on a 13 X 2.5 cm silica gel column (1:l CH2Cl,/hexvacuum. The residue was extracted with benzene and filtered through anes). The orange band was collected. Solvent removal afforded an a 2-in. silica gel plug. The benzene was removed by rotary evaporation, orange powder (0.095 g, 0.132 mmol, 82%), mp 194-197 OC dec, which and the resulting orange oil was chromatographed on a 13 X 2.5 cm silica was shown by 'H N M R (CSHs resonances) to be a (94 f 1):(6 f 1) gel column (1:l CH2C12/hexanes). The orange band was collected, and 5k/5t m i x t ~ r e . ' ~Spectroscopic data are given in Tables 111 and VI. solvent was removed under oil pump vacuum to give 0.078 g (0.118 Preparation of (SS,RR)-[(q-C,Hs)Re(NO)(PPh3)(CHmmol, 87%) of 4t as an orange powder, mp 183-185 OC dec. Spectro(PMe3)C6H~)]+PF6-(6k). To a -78 OC solution of 2k (prepared from scopic data are given in Tables 111 and VI. 0.165 mmol of 1 as described above) was added 0.020 mL (0.014 g, 0.197 Preparation of ( S S ,RR) - ( r)-C,HS)Re(NO) (PPL,) (CHmmol) of PMe,. After 15 min at -78 "C, the reaction was allowed to (CH&H&H5) (5t). Benzylidene 2t (0.309 g, 0.397 mmol) was diswarm to room temperature while solvent was simultaneously removed solved in 40 mL of CH2C12in a Schlenk flask and cooled to -78 O C . under oil pump vacuum. The residue was taken up in CH2C12. DiaThen 0.330 mL of C&CH2MgCI (1.8 M in THF) was added dropwise. After 20 min at -78 OC, the reaction was allowed to warm to room temperature while solvent was simultaneously removed under oil pump ( 5 2 ) 'H NMR analysis was conducted at this stage to assay diastereomer vacuum. The residue was extracted with benzene and filtered through purity. All t isomers were 299% pure. a 2-in. silica gel plug. The benzene was removed by rotary evaporation, (53) Recrystallization of this mixture from CH2C12/hexanes over the and the resulting orange oil was chromatographed on a 13 X 2.5 cm silica course of several days in the freeze afforded pure k isomer. For 3k, it was gel column (1:l CH2C12/hexanes). The orange band was collected, and necessary to halt the crystallization after 10-2096 recovery in order to prevent solvent was removed under oil pump vacuum to give 0.259 g (0.358 cwrystallization of 3t. Table VI. Summary of 16-eV Mass Spectral Data
,
A -
~-
1
*
,,
,
4878 J. Am. Chem. Soc., Vol. 104, No. 18, 1982
Eisenstein, Gladysz, et al.
stereomerically pure 6k was obtained as an orange powder (0.099 g, 0.019 mmol, 59%) were obtained. The 2t-ad, content was assayed as 0.116 mmol, 70%), mp 122-123 "C dec, by allowing ether to slowly described in the preceding paragraph (with identical results). diffuse into this solution in a freezer. Spectroscopic data are given in Reaction of 7t with Ph,CtPFc. To a septum-capped NMR tube was Table 111. added 0.016 g (0.024 mmol) of 7t in 0.300 mL of CD2CI2. The tube was An 'H NMR monitored experiment (see isomerization rate studies) then cooled to -78 OC, whereupon 0.010 g (0.025 mmol) of Ph,CtPFc was conducted on a 0.024 mmol scale in CD2C12at -78 OC. Addition in 0.140 mL of CD2CI2was added via gas-tight syringe. The tube was of 0.005 mL (0.049 mmol) of PMe, to 2k gave an orange solution, which quickly transferred to a -65 OC N M R probe. A IH NMR spectrum upon warming to room temperature was shown by integration of the indicated the exclusive formation of 2t and Ph,COCH, (6 2.91). A small CJHJ resonances to be a (94 f 1):(6 f 1) 6k/6t mixture. amount of starting 7t remained. No products derived via hydride abPreparation of (S3,RR)-(?-C5H5)Re(NO)(PPh3)(CH(OCH3)C6H5) straction from 7t8formed. A small resonance at ca. 6 3.3 was tentatively (7k). To a -78 "C solution of 2k (prepared from 0.350 mmol of 1 as assigned to C H 3 0 C H 3byproduct. described above) was added a solution of CH,ONa generated from Na Reaction of 7k with Ph,CtPFc. To a septum-capped IH NMR tube (0.20 g, 8.7 mmol) and anhydrous CH,OH (8 mL). After 10 min at -78 was added 0.020 g (0.030 mmol) of a 90:lO 7k/7t mixture in 0.300 mL "C, the reaction was allowed to warm to room temperature while solvent of CD2C12. The tube was then cooled to -78 OC, whereupon 0.018 g was simultaneously removed under oil pump vacuum. The resulting (0.046 mmol) of Ph3CtPF6' in 0.150 mL of CD2Clz was added via residue was extracted with benzene (ca. 200 mL) and dried over MgSO,. gas-tight syringe. The tube was quickly transferred to a -70 "C N M R Solvent was removed from the filtrate; IH NMR analysis of the residue probe. A IH N M R spectrum indicated the formation of a (90 f 1):(10 indicated a (78 f 1):(22 f 1) 7k/7t ratio. Diffusion recrystallization f 1) ratio of 2k/2t and Ph3COCH3(6 2.91). No other organometallic of the residue from CH2CI2/hexanesin the freezer gave orange prisms products were present; a very small 6 3.3 resonance was noted. (0.079 g, 0.119 mmol, 35%), which were shown by IH N M R (C5H5 Calculations. All calculations were of the extended Huckel type,55 resonances) to be a (90 f 1):(10 f 1) 7k/7t mixture, mp 155-172 "C with a weighted Hij formula.56 The ligand geometry about rhenium was de^.^, Spectroscopic data are given in Table 111. Anal. Calcd for assumed to be octahedral, and the following distances (A) were used as C31H,9N02PRe: C, 56.01; H, 4.40. Found: C, 55.93; H , 4.51. input: Re-C, 2.10; Re-N, 1.78; N-0, 1.19; Re-P, 2.36; Re-C5H5 A 'HNMR monitored experiment (see isomerization rate studies) was (distance to carbon), 2.33; C-C (C5H5), 1.40; C-H, 1.09; P-H, 1.44. conducted on a 0.019 mmol scale in CD2CI2 at -78 "C. A CD,ONa Standard parameters were employed for carbon, nitrogen, oxygen, and solution that was generated from Na (0.018 g, 0.782 mmol) and CDBOD hydrogen.55 Those utilized for the other atoms were as follows: Re 6s, (0.300 mL) was added to 2k. Reaction was complete within 3 min as Hi, = -9.36 eV, { = 2.398; Re 6p, Hi, = -5.96 eV, { = 2.372; Re 5d, Hi, assayed by a 'H N M R spectrum at -70 "C. A (85 f 1):(15 f 1) = -12.66 eV, { = 5.343 (coefficient = 0.6359) and 2.277 (coefficient = 7k-d3/7t-d, mixture formed. 0 . 5 6 7 7 ) ; P 3 s , H i i = - 1 8 . 6 e V , ~ = 1 . 6 ; P 3 p , H i , = - 1 4 e V , ~ = 1 . 6The . model nucleophile, H-,was given Hii = -10 eV and ( = 1.3. Reaction of l-a-dl-twith Ph3CtPF6-. Preparation of 2t-a-dl. To a 100" Schlenk flask were added 0.21 1 g (0.332 mmol) of I-a-d,-t and Acknowledgment. This investigation was supported a t U C L A 20 mL of CH2CI2. The resulting solution was cooled to -78 OC, and 0.135 g (0.349 mmol)of Ph3C+PF6-was added as a solid. The resulting by the Department of Energy (alkylidene synthesis) and the NIH yellow solution was stirred at -78 OC for 30 min and then allowed to ( G M 29026-01; asymmetric induction) and a t t h e University of warm to room temperature. After an additional 30 min, solvent was Michigan by t h e donors of t h e Petroleum Research Fund, adremoved under oil pump vacuum. The residue was extracted with hexministered by the American Chemical Society. W e warmly thank anes to recover the triphenylmethane. The hexanes were removed from Professor Roald Hoffmann (Cornel1 University) for his interest the extracts via rotary evaporation, and the residue was recrystallized in this study a n d several stimulating discussions. Helpful sugfrom hot 95% ethanol. Fluffy white crystals of triphenylmethane (0.059 gestions from Professors F. A. L. A n e t and D. J. C r a m , Dr. C. g, 0.241 mmol, 73%) were obtained. Analysis of the m / e 244:245 peak B. Knobler, a n d A. T. Patton ( U C L A ) a r e also gratefully acratio in the 16-eV mass spectrum (including comparison to an authentic knowledged. Crystal structure determinations a n d FT NMR sample) indicated a natural abundance Ph3CH/Ph&D ratio. measurements m a d e use of equipment obtained via NSF DeThe residue remaining after hexane extraction was taken up in CH2C12 and filtered through a 2-in. silica gel plug. The CH2CI2was then repartmental instrumentation grants. W.A.K. thanks t h e Regents moved by rotary evaporation, and the residue was diffusion recrystallized of t h e University of California for a Fellowship. from CHCl,/petroleum ether (bp 35-60 "C). Yellow prisms of 2t-aRegistry No. 1, 71763-28-5; l-a-dl-k, 82399-52-8; l-a-dl-t, 82399d,.CHCI, formed, which were collected by filtration and dried (0.195 g, 53-9; 2k, 74540-78-6; 2t, 74561-64-1; 3k, 82374-39-8; 3t, 82399-54-0; 0.217 mmol, 65%; mp 215 OC, de^).^^ 4k, 82374-40-1; 4t, 82399-55-1; 5k, 82374-41-2; 5t, 82399-56-2; 6k, A 'H NMR monitored reaction in CD2C12was conducted analogously 82374-43-4; 6t, 82399-58-4; 7k, 76821-65-3; 7t, 76770-56-4. to the immediately following experiment and showed the sequential formation of 2k-a-dl (6 5.89) and 2t-a-d, (6 6.08). Supplementary Material Available: Figure 8, numbering of Reaction of 1-a-d,-k with Ph,C+PFc. To a septum-capped N M R atoms in ac- [(q-C,HS)Re(NO) (PPh3)(=CHC6Hs)]+PF6-.cHc13 tube was added 0.020 g (0.032 mmol) of diastereomericallypure 1-a-dl-k in 0.400 mL of CD2C12. The tube was then cooled to -78 OC, whereupon ( Z t C H C l , ) ; Figure 9, crystal packing diagram of 2t.CHC13 a s 0.01 34 g (0.035 mmol) of Ph,C+PF6- in 0.150 mL of CD2CI2was added viewed down axis A; Table VII, summary of crystallographic data via gas-tight syringe. The tube was quickly transferred to a -73 OC for 2t-CHC13; Table V I I I , bond a n d short interatomic distances N M R probe. A spectrum indicated the complete consumption of l-ain 2t.CHC1,; Table IX, bond angles in Zt-CHCl,; Table X, atomic dl-k and the presence of 2k (6 16.08, 5.89) and some PhpCH (6 5.59). coordinates of Zt-CHCl, (atoms refined isotropically); Table X I , Upon warming to room temperature in the N M R probe, 2k disappeared atomic coordinates of 2t.CHC13 (atoms refined anisotropically); as 2t appeared. After several hours at room temperature, the +Re= Table XII, calculated and observed structure factors for ZtCHCI,; CHC6H5(6 15.30) and C5H5(6 6.05) resonances of 2t were integrated Figure 10, numbering of a t o m s in (sS,RR)-(tl-C,H,)Re(NO)repeatedly. From these data, a (92 f 2):(8 & 2) ratio of 2t-a-do/2t-a-d1 (PPh3)(CH(CH2C6H5)C,H5)-CHIC], (5tCH2C12); Figure 11, was calculated. The contents of the NMR tube were applied to a preparative silica gel crystal packing diagram of 5t-CH2C1,a s viewed down axis A; TLC plate, which was then eluted with 9010 ethyl acetate/hexanes. The T a b l e X I I I , s u m m a r y of crystallographic d a t a for 5t.CH2Cl2; UV-active triphenylmethane band (I?, -0.7) was isolated and extracted Table X I V , bond and short interatomic distances in 5t.CH2C12; with CH2C12. The eluent was concentrated to a residue, which was then Table XV, bond angles in 5tCH2C12;Table X V I , atomic coorrecrystallized from hot 95% ethanol to give pure triphenylmethane. dinates of 5t.CH2C12 (atoms refined isotropically); Table X V I I , Analysis of the m / e 244:245 peak ratio in the 16 eV mass spectrum atomic coordinates of 5t.CH2C12 (atoms refined anisotropically); indicated a (89 4 I):(] 1 + 1) Ph,CD/Ph,CH ratio. The origin of the Table X V I I I , calculated a n d observed structure factors for 5tTLC plate was extracted with CH2CI2. The extract was concentrated CH2C12(65 pages). Ordering information is given on any current to a residue, which was then diffusion recrystallized from CHC13/pemasthead page. troleum ether (bp 35-60 "C). Yellow crystals of 2t-CHCl, (0.017 g,
-.
-
(54) It should be noted that the sequence (q-C,H,)Re(NO)(PPh,)(CD3)'JSb l-a-d2 [2k-a-dl] 2t-a-dl (all by procedures analogous to those given for unlabeled compounds above) constitutes a considerably easier and more direct synthesis of 2t-a-dl. +
(55) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. Hoffmann, R.; Lipscomb, W. N. Ibid. 1962, 36, 2179; 1962, 37, 2872. (56) Ammeter, J. H.; Biirgi, H. B.; Thibeault, J. C.; Hoffmann, R. J . Am. Chem. SOC.1978, 100, 3686.
